One technique used to protect jet aircraft against missile attack is an electronic decoy, which is towed behind the aircraft by a cable. Due to the high performance of these aircraft and the significant trailing vortices created by the aircraft wings, particularly during high “G” manoeuvres, the method by which the cable is terminated and connected to the decoy is fundamental to the successful operation of the towed decoy. The cable termination has to transmit those forces acting on the decoy in flight (aerodynamic drag and inertia) into tow cable.
The decoy's flight performance follows from its aerodynamic design, which to a large extent is determined by the relative positions of the centres of pressure and mass of the decoy. With a highly stable decoy, the decoy's centre of mass follows the tow cable, so that the cable termination transmits the loads from the decoy to the cable with little off-axis deviation. However, with a less stable decoy, or in a very high vortex environment or during high “G” manoeuvres, the axis of the decoy can fluctuate or deviate significantly with respect to the axis of the cable, typically up to a cone angle of plus or minus 90 degrees. In these circumstances, either the cable termination has to flex over this extremely wide angle of movement, or the tip of the tow cable itself has to flex over this range. Modern materials such as Kevlar (Kevlar is a trademark of EI Dupont de Nemours and company) have a very high strength to weight ratio and therefore are ideal for use as tow cables. However, even such modern materials cannot sustain local bending in a highly dynamic environment, and the rapid and persistent flexure of the cable near to the cable termination in a high vortex environment can lead to catastrophic failure. Specialist cable terminations have been proposed, but these generally have a limited angle of articulation, typically less than a cone angle of plus or minus 30 degrees. Techniques to relieve cable strain, to compensate for this low articulation, have been used, but these methods are insufficiently robust significantly to extend the life of the cable/cable termination arrangement. A low angle of articulation at the cable termination results in the tow cable flexing near the tip of the cable, resulting in the failure of the tow cable and the loss of the decoy. This mode of failure places a severe restriction on the flight envelope of the aircraft when towing a decoy.